This invention relates generally to the technology of coating clays, and more specifically relates to an improved process for preparing high grade kaolin clays for coating applications.
In the commercial processing of most "normal" kaolin clays, it is common practice to remove a portion of the discoloring iron contaminants by leaching. In order, in particular, to reduce discoloration caused by iron compounds, it is common to subject an aqueous slurry of the clay to a leaching and flocculating operation, pursuant to which some of the oxides of iron are converted by reduction to soluble ferrous compounds. This is most commonly accomplished by use of either zinc or sodium salts of hydrosulfurous acid as leachants, and the use of aluminum sulfate and/or sulfuric acid as flocculating agents. This procedure is normally followed by a dewatering or filtration step, dispersion, and drying.
Some kaolinitic clays, however, in addition to containing iron in a form which can be solublized by leaching, include iron in the form of iron pyrites; that is, in the form of iron sulfide. These sulfide contaminants do not become readily soluble under ordinary leaching conditions, and the leached clay therefore retains a gray, unacceptable color.
In the foregoing connection, it may be noted that it has been known for some time that magnetically attractable contaminants can, to a degree, be removed from aqueous slurries of kaolin clays by imposing on the slurry a high intensity magnetic field gradient. The forces produced upon the particles by the magnetic field gradient, effect differential movements of mineral grains through the field, in accordance with the magnetic permeability of the minerals, their size, mass, etc. Where, as discussed herein, the contaminants take the form of the iron pyrites mentioned, it has however been found that the techniques of magnetic separation may not be advantageously employed, in that the magnetic attractability of the said contaminants is unfortunately too low to yield worthwhile results.
In accordance with the foregoing, it may be regarded as an object of the present invention to provide a method for processing kaolinitic clays which contain discoloring iron-based contaminants in the form, at least partially, of iron pyrites, which results in removal of a high proportion of the contaminants, thereby yielding a high brightness clay suitable for coating applications.
It is a further object of the present invention, to provide a method for the processing of kaolinitic clays including iron contaminants in the form of iron pyrites, which by enhancing the magnetic susceptibility of the sulfide contaminants, augments the effectiveness of magnetic separation techniques, thereby yielding a high brightness clay.
It is a still further object of the present invention to provide a method for the processing of kaolinitic clays, including iron pyrites as discolorants, which so alters the said contaminants as to facilitate their removal by a combination of magnetic separation and leaching steps.